Chest deformation low poly
That should be a weightpainting problem.
As far as I can see you have more than enough mesh for the deformation in that armpit area and your geometry looks clean, too. 
Weightpainting can be a pain in the butt and takes a lot of time to get it right.
I never weightpainted in Blender, but usually 3d programs can display the weights in different colors so you can see more clearly what's going on with your weights.

Hope this helps you a little :)